The main problem with IMSLP's indexing is the practice known as "hit and run" uploading - a contributor will upload a big publication comprising twenty or more works (or sometimes over a hundred!) without providing any information about the contents, other than the title of the volume and a one line description. Recently, the 33 volumes of the Haberl edition of Palestrina went up, so with what seems like little assistance at all (Richard Mix has made a few helpful edits), I've been diligently adding contents to each of those pages and compiling the list of which piece is on which page. In the meantime, other tasks slowly creep to the back-burner. In general, pages without contents listings are particularly awkward for the collections of early music that are being uploaded, since a relatively smaller proportion of IMSLP contributors seem assured or knowledgeable of repertoire say pre-1650.
The paucity or inconsistencies of the Monteverdi pages at IMSLP are partly my fault - although I've added the Stattkus-Verzeichnis to the list of compositions page, I've only gotten through about half the secular music to add contents lists to each individual work page, and hadn't gotten around to any of the big sacred collections (1610, 1638, 1651).